The present invention relates to a process for the treatment of fly ash accumulated in the flue gas filter of the soda recovery unit used for the recovery of chemicals in the manufacture of sulfate cellulose, so as to separate commercially exploitable chemicals therefrom. According to the invention, a process is provided wherein no waste is formed from the fly ash; all of its components are exploited.
For reasons of environmental protection, an aim in connection with the manufacture of sulfate cellulose is to attain as complete recovery of chemicals as possible in order to reduce effluent emissions. A soda recovery unit in which the waste liquor it of the cellulose mill is burned is used for the recovery of chemicals. In the flue gas filter of the soda recovery unit there accumulates a very fine-grained fly ash, which is composed of a solid-state salt mixture. The composition of the fly ash is typically:
In other respects the fly ash is rather pure, although it does contain a small amount of water-soluble calcium as an purity. In addition to sulfate, the fly ash contains only carbonate in a significant amount.
There are previously known several processes by which potassium and chloride, which are regarded as detrimental substances in terms of further use, are removed from the fly ash and are thereafter released along with the effluents into water-courses. It is an object of the present invention indeed to recover, in a commercially exploitable form, all of the chemicals accumulating in the fly ash from the chemicals cycle of sulfate cellulose manufacture. It is a particular object of the invention to provide a process for treating the raw material in such a manner that hardly any substance to be considered waste is formed, and that also the remaining chloride-containing solution is so pure that it can be used as a raw material of an electrolysis process, for example, in the production of chlorine alkali or chlorate. According to the invention, this is accomplished in the manner described in the accompanying claims. FR patent application 2 761 088 describes a process for the recovery of sodium sulfate from fly ash formed in connection with the manufacture of sulfate cellulose. In this process, the fly ash is dissolved in water, carbonates are removed from the obtained solution by a sulfuric acid treatment, and the obtained sodium sulfate is separated from the solution by crystallization. In the process according to the French patent application, the remaining solution is in part recycled to the process and in part released into the effluents. By this process, the sodium carbonate present in the fly ash is converted to sodium sulfate, which can be recovered in an almost pure form. However, potassium sulfate and sodium chloride are still left in the unexploitable solution passing into the effluents. In this process, hydrochloric acid cannot be used for breaking down the carbonate, since it would only increase the amount of sodium chloride waste.
However, it is not profitable to crystallize all of the sodium sulfate, since this is technically cumbersome and would therefore be expensive; instead, the mother liquor from the crystallization can be directed to a subsequent purification step, wherein a portion of the remaining sodium sulfate is separated by precipitation. According to the process disclosed in JP patent 960196332, the sulfate is precipitated by adding calcium chloride to the above-mentioned mother liquor, whereupon calcium sulfate (gypsum) which precipitates and can be removed by filtration is obtained as a product.
In the above-mentioned precipitation of gypsum, another product formed is sodium chloride NaCl, which contains as impurities both calcium and sulfate in such amounts that they hamper the use of the formed NaCl as a raw material in electrolysis. Thus, if it is desired to use this NaCl salt as a raw material in electrolysis, it must be purified further.
U.S. Pat. No. 3,833,462 describes a process wherein sodium chloride is separated from fly ash for further exploitation. In this process, the sodium carbonate is converted to sodium sulfate by means of sulfuric acid, and the solid sodium sulfate obtained in the process is separated from a sodium chloride solution. However, the Na chloride solution contains potassium sulfate as an impurity, and thus it cannot be used directly as a raw material in the chemicals industry.
In the process according to the present invention, the aim is to obtain as high a sodium chloride yield from fly ash as possible. For this purpose, the carbonate present in the fly ash is broken down with hydrochloric acid, whereupon a solution which contains in the main Na chloride and sulfate is obtained as the result. In the process according to the invention, the Na sulfate need not necessarily be crystallized out from the solution; the entire solution can be fed directly to the precipitation step. In this process, a chloride, for example, calcium chloride or barium chloride, is used for the precipitation of the sulfate. The calcium or barium sulfate precipitate obtained by chloride precipitation can be separated by filtration. Thereupon there is left a mother liquor which contains mainly Na chloride.
When sulfate is precipitated with barium chloride in accordance with the invention, Ba sulfate is obtained, which is a useful product and can be used, for example, in medicine as a contrast material in radiography and in the paper industry as a pigment and a filler. Barium sulfate is quite poorly soluble, and thus the mother liquor is rather devoid of sulfate and barium. Thus there is left in the solution, dissolved, Na chloride and a small amount of K chloride. This chloride solution is so pure that it can be used as a raw material, for example, in the preparation of chlorine and chlorate.
In another embodiment of the invention, the sulfate may first be precipitated with calcium chloride, and the calcium sulfate is separated from the solution. The obtained mother liquor still contains as impurities too much calcium and sulfate for the liquor to be used directly as a raw material in the electrolyte industry. The mother liquor has to be purified further by the removal, by barium chloride precipitation, of the sulfate left in the solution. In addition, any excess calcium can be removed from the mother liquor by adding to it a suitable amount of sodium carbonate.